Often in the production of ceramic materials first a porous body is produced by a thermal process step, such as, for example, sintering or pyrolysis, and is then compacted. This compaction can take place by pressing, that is, without adding another substance, or by one or more infiltration or impregnation steps, while the infiltration agent can remain unchanged and at least partially fills only the (open or accessible) pores, or the infiltration agent reacts at least in one part with at least one of the components of the porous body, with the formation of at least one chemical compound.
A field in which this method has acquired technical importance is the production of ceramic material based on silicon carbide, first a porous carbon body being produced which then, by a reaction with liquid silicon or a silicon-containing melt with for the most part preservation of the geometrical shape and dimensions (so-called “near net shape method”) of the carbon body, is converted into a body which largely consists of a silicon carbide which has been formed by the reaction of carbon and silicon. At the conventionally used temperatures above the melting point of silicon up to 1700 ° C., the beta-modification of the silicon carbide which is stable at these temperatures is formed.
There are various ways of introducing liquid silicon into porous, carbon-containing bodies. U.S. Pat. No. 5,432,253 discloses placing the porous body which is to be impregnated jointly with silicon in lump form on a carbon cloth (fabric or felt) and heating everything together to the infiltration temperature, that is, a temperature at which the silicon is molten liquid and has a low enough viscosity to penetrate by way of the cloth into the porous body and to be distributed in it.
Another method is known from U.S. Pat. No. 4,626,516 in which a mold with infiltration holes and a reservoir with elementary silicon is used; a porous body is placed in the mold, and the holes are provided with carbon wicks, the wicks being in contact with the porous body and the reservoir. At the operating temperature above the melting point of silicon, the silicon migrates through the wicks into the porous body.
U.S. Pat. No. 4,737,328 discloses covering a porous body with a powder mixture of silicon and boron nitride, and heating the arrangement above the melting point of the silicon, its penetrating into the body, and the boron nitride powder then being brushed off. Another method is covering a porous carbon body with a moldable and hardenable mass of silicon powder and a binder according to patent application WO1982/04248 or patent application EP 0 995 730 A1 . The advantage there is supposed to be that the silicon is delivered exactly in the required amount. Finally, patent application DE 102 12 043 discloses placing the article which is to be infiltrated with silicon on wicks of carbonized wood, these wicks with their lower end dipping into a metal melt.
The problem which is common to all these methods is that the silicon melt is always supplied from the surface of the body to be infiltrated. Here, in the vicinity of the entry point there is always a great excess of silicon, while much less silicon is transported into the regions farther away. For two-dimensional delivery as in applications WO 1982/04248 or EP 0 995 730 A1 . liquid silicon is supplied over the entire covered surface, but a gradient forms over the thickness of the body to be infiltrated. These gradients lead to inhomogeneity in the infiltrated and potentially reacted bodies which occasions an intolerable imbalance, especially for parts turning at high speed, such as brake disks or clutch disks.